Construction and renovation in cold climates requires that temporary heat be provided so that workers can function effectively. Conventionally, such temporary heat has been provided by construction heaters that include a burner burning a fuel, typically propane, and a fan to circulate the air warmed by the burner in the area to be heated. Such direct combustion creates exhaust fumes in the area heated that can present health hazards to workers. As well, combustion creates moisture leading to increased relative humidity in the work area. Construction materials absorb the moisture and can be damaged, or later warp when the relative humidity returns to a normal level.
For this reason heat exchangers have been used whereby the exhaust and moisture created by combustion are kept separated from an air stream that is heated and directed into the construction area. Such a heat exchanger system is generally disclosed for example in United States Patent Application No. 2003/0056390 of Adrian. Circulating liquid construction heating systems are also known that include a liquid heater that heats liquid, typically a mixture of water and glycol, and a pump that circulates the heated liquid through conduits to portable remote heating units located in one or more construction areas. The remote heating units typically comprise a coil and a fan. The warmed liquid circulates through the coil and the fan blows air through the coil to be heated and circulated through the area where the unit is located. Moisture and exhaust fumes created by combustion at the liquid heater are thus kept remote from the areas being heated.
The liquid pressure generated by the circulating pump in a typical circulating liquid heating system is relatively low at about 30-40 psi, similar to the pressure in a water supply system. Typically as well, conventional circulating liquid heating systems are configured such that the intake of the pump is connected to the output of the liquid heater. The pump thus draws liquid through the heater instead of pushing it through the heater, and so the liquid in the heat exchange section of the heater is under negligible, if any, positive pressure. Liquid in the heat exchange section is typically directly connected to a permanently open atmospherically vented expansion tank, located at an elevation above the circulation pump, such that as the liquid temperature rises extra liquid volume can flow into the expansion tank, and if the temperature falls, or if there is a leak in the conduits, or extra liquid is required during set-up, extra liquid volume can flow from the expansion tank back into the conduits. Thus the conduits are kept full, and only atmospheric pressure is present in the heat exchange section of the liquid heater.
Such a configuration results in an inherently safe system. Such liquid heaters are not required to meet regulated safety specifications or be regularly inspected as is the case with boilers and pressure vessels. For regulation purposes, the liquid heater is classed essentially as a water heater, since the pressure in the heat exchange portion thereof is at zero pressure relative to the atmosphere, and further pressures developed by the circulating pump do not exceed that of a water supply system.
In many areas, high rise buildings of 30 to 70 stories or more have been in existence for many years, and renovation of such high rise buildings is becoming common, requiring temporary heating. Construction of such buildings also requires temporary heating to allow construction to proceed during cold weather.
Using conventional circulating liquid heating systems during construction and renovation of high rise buildings presents certain challenges compared to use in buildings that are relatively close to the ground. Most conveniently the liquid heater will be located at ground level for ease of fuel delivery. The ground level liquid heater location also facilitates the addition of liquid to the circulating liquid system as will be required when setting up the system, and when the conduits are lengthened, or more remote heating units are added to the system. In addition such liquid heaters can be quite cumbersome, and difficult to move to upper areas of a building. In order to operate such circulating liquid systems with the liquid heater on the ground, it is required to circulate the liquid under high pressure conditions caused by the high elevation of the remote heating units.
While considerable pressure is required to raise the heated liquid to the upper floors of a high rise building, once the conduits up to the remote heating units are full of liquid, the pressure on the downward flowing portion of the conduits will substantially balance the pressure on the upward flowing portion thereof, and the conventional circulating pump generating a pressure of about 35 psi, such as would be used in a conventional circulating liquid heating system, would provide sufficient pressure to circulate the liquid in the full system. The result however is that the pressure in the liquid at the bottom portion of the conduits will be that developed by the head of liquid in the conduits on the return side of the pump, and on the supply or output side of the pump the pressure will be that developed by the head of liquid in the conduits plus the pump pressure.
Every 2.3 feet of water in a vertical conduit generates about one pound per square inch (psi) of pressure. Thus where the remote heating units are 230 feet above a liquid heater at ground level, the pressure in the liquid at the bottom of the conduits will be about 100 psi. Similarly where the remote heating units are 690 feet above a liquid heater at ground level, for example on the 69th story of a building, the pressure in the liquid at the bottom of the conduits will be about 300 psi. The conventional circulation pump generating a 35 psi pressure differential between the input and output would suffice to circulate the liquid through the conduits, and the pressure at the input of the pump would be 300 psi, and at the output about 335 psi.
Flexible conduits of the type used with conventional circulating liquid heating systems can readily be made to withstand pressures of 300 psi or more, and the operation of the remote heating units of such a system is not adversely affected by the elevation as long as sufficient liquid flows through their coils. The elevation does however adversely affect the liquid heater of conventional systems.
Thus while such a conventional circulating liquid heating system could theoretically be used in a high rise application, the expansion tank would need to be sealed from the atmosphere, or alternately raised above the highest level at which remote heating units will be located. Once the conduits to the remote heating units were filled, the liquid flowing through the liquid heater would be pressurized to a considerable pressure resulting from the high elevation of the remote heating units and conduits connecting them. Considerable expense would be involved in re-designing such heaters then to meet the safety specifications for pressure vessels.
Steam heating is popular for use in large buildings. In densely populated urban areas, such as where high rise buildings are typically located, heat is often provided by a steam generation utility that transports steam from a central location to buildings in an area that might encompass several city blocks. Steam heat is thus available simply by connecting the steam utility pipes to a suitable steam heating system in an existing or newly constructed building, much the same as connecting to water or electrical utilities. Where steam heat is available, the cost is generally favorable compared to other heating options.
Steam heating systems in buildings typically comprise steam supply pipes and condensate return pipes connected to a steam source such as a steam boiler or steam utility supply system. On each floor a distribution network of steam supply pipes, radiators, condensate return pipes, vents, and steam traps are properly sloped and configured so that steam will flow into the radiators, condense, and flow downhill back to the steam source as water. All these fixtures will be removed when, for example, the renovation involves gutting the building interior completely. Commonly during renovations, such a steam heat source might be available but the precision required to lay out a properly functioning temporary distribution system for the steam has made it impractical to use the steam heat for temporary heating during renovations. Similarly during new construction the steam heat source may be available, however it has not been practical to utilize it during construction.